1. Field of the Invention
The present invention relates to an anamorphic single lens used for elevating the transfer efficiency of beams or other similar purposes in an optical scanning system, particularly in a post-objective type optical scanning system in which laser beams are bundled and turned into a focusing pencil of rays and thereafter deflected by a polygonal mirror.
2. Description of the Prior art
In recent years, there has been studied and developed a post-objective type optical scanner in which laser beams are turned into a focusing pencil of rays and thereafter deflected by a polygonal mirror. Such an optical scanner has been manufactured as a commodity through research and development, thereby permitting a realization of miniaturization in size and reduction in cost.
In the overall optical system, however, since the focal length in the main scanning direction is ten or more times as large as that in the sub-scanning direction, in order to have an equal spot size on the image surface in the main scanning and sub-scanning directions, the configuration of an aperture stop placed before the polygonal mirror needs to be a rectangle or a long ellipse having a ratio of its width in the main scanning direction to that in the sub-scanning direction of 10:1 or higher. For this reason, the necessary transfer efficiency of the beams cannot be obtained merely by collimating the semiconductor laser beams. For solving this problem, a method utilizing prisms has been available as shown in FIGS. 6, 7(a) and 7(b).
FIG. 6 shows an optical scanner utilizing prisms, where reference numeral 41 denotes a semiconductor laser; 42 a collimating lens; 43 and 44 prisms; 45 an aperture stop; 46 a focusing lens; 47 a cylindrical lens; 48 a mirror; 49 a polygonal mirror having cylindrical surfaces; 50 a compensating lens; and 51 a photosensitive drum.
Referring to FIGS. 7(a) and 7(b), the semiconductor laser 41 for emitting light beams is arranged such that the direction of the beams emitted at a larger divergence angle, is coincident with the main scanning direction. The light beams emitted from the semiconductor laser 41 are turned into parallel beams through the collimating lens 42, having an intensity distribution in the form of an ellipse with a ratio of about 3:1 of the beam diameter in the main scanning direction to that in the sub-scanning direction. Then the beams are contracted in diameter only in the sub-scanning direction by the prisms 43 and 44, thereby turning the beams into parallel beams having an intensity distribution ratio of about 10:1 in diameter in the form of an elongated ellipse. Thereafter, the beams pass through the aperture stop 45, focusing lens 46 and the cylindrical lens 47, and then the beams are applied to the polygonal mirror 49 by way of the mirror 48.
As a single lens for performing the functions of the collimating lens and prisms, there is known a lens for use in an optical disk as is disclosed in the Japanese Patent Laid-Open (Unexamined) 61-254915 (published in 1986).
However, the method of using prisms results in complex structure, posing problems of high cost and difficulty in miniaturization. The lens disclosed in the above Japanese Patent Laid-Open Publication 61-254915 has a toric surface represented simply by a radius of curvature, posing a problem of being impractical because of large residual spherical aberration. Further, since such a lens has a function of reshaping flat beams into circular ones, there is a problem in that it is ineffective even when the lens is used in a post-objective type optical scanner.